示例用法：
public static void main(String[] args) {
//Semaphore s = new Semaphore(2);
//Semaphore s = new Semaphore(2, true);
//允許一個(gè)線程同時(shí)執(zhí)行
Semaphore s = new Semaphore(1);
new Thread(()->{
try {
s.acquire();
System.out.println("T1 running...");
Thread.sleep(200);
System.out.println("T1 running...");
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
s.release();
}
}).start();
new Thread(()->{
try {
s.acquire();
System.out.println("T2 running...");
Thread.sleep(200);
System.out.println("T2 running...");
s.release();
} catch (InterruptedException e) {
e.printStackTrace();
}
}).start();
}
